In recent years, researches and developments of secondary batteries such as lithium ion secondary batteries and nonaqueous electrolyte secondary batteries as high energy density batteries have gathered pace. Secondary batteries are expected as power sources for vehicles such as hybrid automobiles and electric automobiles, or for uninterruptible power supplies for mobile phone base stations. In particular, all solid-state lithium ion secondary batteries are actively researched as automotive batteries, and their high safety has attracted attention.
Compared with a lithium ion secondary battery using a nonaqueous electrolyte, the all solid-state lithium ion secondary battery uses a solid electrolyte, and therefore, there is no fear of ignition. However, a high capacity all solid-state lithium ion secondary battery has not yet been put into practical use at present. One of the reasons for this is that the interface resistance between the solid electrolyte and the active material is high. Since both are solid, it is relatively easy to bond the solid electrolyte and the active material by applying heat. However, since the active material swells and shrinks due to insertion and extraction of lithium, the active material is peeled off from the solid electrolyte when repeatedly charged and discharged, so that good cycle performance cannot be obtained in some cases.
Thus, it is necessary to suppress an influence of swelling/contraction of the active material, and to form a good interface between the solid electrolyte and the active material.